September Equinox

Happy September Equinox, everyone!

Due to differences between the calendar and tropical years, the autumn equinox often happens between September 21-24. This year, it's on September 22.

Where did the term "equinox" come from, you may ask? Well, the word "equinox" is descended from Latin words meaning "equal nights". That's because for most latitudes, as shown in the diagram, the duration of the day and the night is about equal. This is because of the Earth's rotation axis becoming perpendicular to the Sun's light. It's not tilting towards or away from the sun at all. The equinoxes are the only time the sun appears to rise in the East and set in the West.  

Here in the Northern Hempishere, where I live, this is the autumn equinox. This equinox marks the end of summer and the start of autumn, in astronomical terms. This is because after this, the Earth's tilt will start to change. For the Northern Hemisphere, the tilt will take us further from the sun, causing colder temperatures. For the Southern Hemisphere, the tilt will be towards the sun, leading to warmer temperatures. As a result, the September equinox marks the beginning of summer for them.